Supply voltage variation compensated servosystem



July 31, 1962 E. v. HARDWAY. JR 3,047,784 SUPPLY VOLTAGE VARIATIONCOMPENSATED SERVOSYSTEM Original Filed Nov. 50, 1959 [dwaro 1 Hardwayg/nINVENTOR. BY%M 1' ZJwM lr ATTORNEYS United States Patent 3,047,784SUPPLY VOLTAGE VARIATION CONIPENSATED SERVOSYSTEM Edward V. Hardway, In,Houston, Tex., assignor to Houston Instrument Corporation, Houston, Tex.Continuation of application Ser. No. 856,075, Nov. 30, 1959. Thisapplication Feb. 2, 1961, Ser. No. 87,054 4 Claims. (Cl. 318-28) Thisinvention relates to improvements in a differential transformer type ofcircuit for measuring displacements.

It has been known to employ a differential transformer to measurephysical displacements of a sensing device. Thus, the sensing device isconnected to the movable core of the transformer and the resultingrelative changes in the voltages or currents of the transformersecondaries are measured to provide an output which is a function of thedisplacement of the core. Heretofore, many of the differentialtransformer circuits used for this purpose have been designed so as toemploy a highly stabilized voltage or current regulated source for theprimary of the transformer. Measurement accuracy has been dependent onthe degree of regulation of the source for the primary and also upontemperature drift effects of the coil windings. These factors haveallowed, at best, accuracies of no better than one percent. Even thisaccuracy cannot be obtained throughout a broad range due to thenon-linearity of the transformer.

It is accordingly one object of this invention to provide a measuringcircuit of the differential transformer type in which temperature drifteffects, non-linearity and the like are compensated for in the read-outcircuit connected to the transformer.

Another object is to provide a differential transformer type measuringcircuit which does not necessarily require a highly regulated input tothe primaries.

Another object of this invention is to provide such a circuit in whichany non-linearity of the transformer is compensated for by employing aratio of signals from the transformer such that non-linearity of onesignal is compensated by a corresponding non-linearity of the othersignal.

In the drawing, which schematically illustrates a preferred embodimentof the invention, a differential transformer is shown as comprising theusual primary 11 and a pair of secondaries 12 and 13 connected inopposition with each other and grounded at their common juncture. Theinput to the primary 11 can be derived from any suitable source whichneed not necessarily be closely regulated. In the preferred embodiment,the source 14 is of relatively low frequency, which may be 400 cyclesper second and preferably is controlled by a tuning fork. The output ofthe source can be amplified 'by voltage amplifier 15 which in turn isconnected to the primary.

As shown in the drawing, a reference amplifier 16 and an error amplifier17 are connected to the differential transformer in such a manner thatthe input to the reference amplifier is the sum of the voltages (e and eof the transformer secondaries and the input to the error amplifier isthe difference in these voltages. Thus, a transformer 18 is connectedacross the secondaries 12 and 13 to give the voltage sum while the inputto the error amplifier is derived from center tapping transformer 18 anddevelops the input signal across resistance 19. Resistance 19 cancomprise a potentiometer so that by adjusting the level of the voltageinput to amplifier 17, the mechanical magnification of the system can bechanged.

It will be noted that any variances in voltages, current 3,047,784Patented July 31, 1962 'ice or phase caused by variations at the sourceor in primary impedance are inherently combined in both signals whichare respectively the inputs to the two amplifiers. Reference amplifier16 is preferably a stable low gain amplifier, while the error amplifier17 is a relatively high gain amplifier.

A summing circuit 20 is connected to the amplifier outputs in order tocompare the amplified reference and error voltages. Thus, the erroramplifier output can be fed to resistance 21 and the reference amplifieroutput to resistance 22. These outputs should be in phase opposition andthis can be accomplished in numerous ways such as by employing anisolation transformer 22 to couple the reference amplifier to resistor22.

Means are provided for varying one of the error and reference voltagesresponsive to a departure from a predetermined relationship between thetwo voltages so as to maintain the voltages in such relationship. Whileany number of suitable means can be employed, the illustrated onecomprises a potentiometer 23 connected across the output of thereference amplifier.

It will thus be seen that a portion of the reference voltage is takenfrom a potentiometer and added in reverse phase to the error voltage ata summing junction 24 which is located at the input to isolationamplifier 25. Whatever the ratio of error voltage to reference voltage,a setting of the potentiometer 23 will exist (within limits) at whichtheir sum will be zero. Since the differential transformer errors havebeen carried through both loops in varied ratios, their net effect atthe summing point is zero. In this connection, the relative gain of thetwo amplifiers 16 and 17 should be such as to give the desired range ofadjustment of potentiometer 23.

At this point, it should be noted that large movements of the core 10aof the differential transformer will result in non-linear outputs; thatis, the error voltage will be non-linear as will the reference voltage.In the circuit shown, a departure of the error voltage from linearity isoffset by a corresponding departure of the reference voltage in anopposite sense. For example, a non-proportional decrease in the errorsignal is matched with a corresponding reduction in the reference signalso as to decrease the error due to non-linear operation of thetransformer.

A zero control voltage can be derived from a potentiometer 26 connectedacross the output of the reference amplifier and this voltage can be fedto summing junction 24 via resistance 27.

From the summing junction, the magnitude of the inequality of thereference and error voltages and zero control voltage are sensed by aphase demodulator 24 and [fed as a D.-C. signal to a servo amplifier 29.A servo motor 30 drives the pick off contact of potentiometer 23 to aposition such that the sum of the error and reference voltages at thesumming junction will be Zero; that is, potentiometer 23 is driven backto null. Therefore, the position of the pick-off for potentiometer 23 isa function of the position of core 10a. The servo motor can beconnected, therefore, to an indicator or a recorder which will thenrecord the movements and positions of the movable core 10a.

It will be apparent that two such circuits as here described can becombined as an X-Y plotter to obtain and record physical displacementssubstantially independently of fluctuations in the output of the currentsource for the differential transformer primary, non-linearity of thedifferential transformer, temperature drift effects of the core windingand other variables tending to cause variations in the differentialtransformer output other than those due to core movement.

This application is a continuation of my co-pending application SerialNumber 6,075, filed November 30, 1959.

are obvious and which are inherent to the apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is oontemplated by and is Within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying'drawing is to beinterpreted as illustrative and not in'a limiting sense.

The invention having been described, what is claimed 1. In an apparatusfor measuring variations in electrical unbalance of the secondaries of adifferential transformer, "a reference and an error amplifier havingtheir respective inputs connected to said transformer secondaries suchthat the input to the reference amplifier is the sum of the voltagesacross the transformer secondaries and the input to the'error amplifieris the voltage difference between the transformer secondaries, a'summiugcircuit connected to the outputs of the reference and the erroramplifiers to compare the amplified reference and error voltages, andmeans for varying one of said voltages responsive to a departure from apredetermined relationship between the two voltages so as to maintainsaid voltages in said predetermined relationship.

2. Inan apparatus 'for determining the position of a core in adifierential transformer, means connectable to the secondaries of saidtransformer to provide a first output which is a function of the sum ofthe voltages across said secondaries and a second output which is afunction of the difierence of the voltages across said secondaries, andmeans for comparing said first'and second outputs and for varying atleast one of them responsive to a departure from a predeterminedrelationship between the two outputs so as to maintain said first andsecond outputs in said predetermined relationship.

3. In an apparatus for determining the core position of a differentialtransformer, a potentiometric rebalance servomechanism having arebal-ance potentiometer connected to the transformer so that thevoltage applied to the potentiometer is a function of the sum of thevoltages across said transformer secondaries and means for comparing thepotentiometer output voltage with the difference of the voltage acrosssaid transformer secondaries and for driving the said rebalancepotentiometer in response to a departure from a predeterminedrelationship between the potentiometer output voltage and saiddifference voltage.

4. In an apparatus for determining the core position of a differentialtransformer, means connected to the secondaries of said transformer tocompare the sum of the voltages across said secondaries with thedifference between the voltages across said secondaries, andservomechanism means for varying at least one of said sum and differencevoltages responsive to a departure from a predetermined relationshipbetween such voltages so as to maintain said voltages in saidpredetermined relationship.

No references cited.

